NEF is the file extension of Nikon RAW image. It stands for Nikon Electronic Format. NEF is used only on Nikon digital cameras. Same as other RAW files such as CR2, NEF also retain everything captured by camera censor, including the metadata. No information is compressed like JPEG. That is why a NEF file typically has much larger size than JPEG. A NEF file can be twice or three times larger than JPEG.
Because no data is compressed, you will not lose the image quality even if you have edited your image over and over. It’s the inverse of JPEG which typically will lose its quality following the editing processes. NEF is a better option if you often perform much editing over your photos. Most professional photographers who use Nikon cameras prefer to use this format over JPEG, regardless the complexity of editing process.
How to open a NEF file
There are two reasons of opening a NEF file. First, you just want to preview the photo without making editing. Second, you want to edit it. You need the different tools for those two purposes. If you just want to open a NEF file without making any edit, then regular image viewer is enough. Today, most default image viewer of major operating systems, be it Windows, macOS and Linux-based have the capability to open a RAW image format with the extension of .NEF.
Conversely, you need a capable image editing tool to open as well as edit a NEF file. Photoshop and Lightroom are still the most popular (if not the best) tools to edit a NEF file. If you are Linux user or someone who love to use free tools, applications like Darktable and Shotwell are nice for playing with a NEF file.